This invention relates to surface acoustic wave (SAW) devices. More particularly, the invention relates to a SAW filter type utilizing a slant reflector pair arranged to provide compensation for temperature dependent signal attenuation which results from instability in the transmission characteristics of piezoelectric material due to temperature dependence.
In order to reduce insertion losses in conventional SAW filters, one approach is to use a "ring" configuration to redirect the bidirectional energy launched in two opposite directions from the transmitting, or input, transducer for being combined at the receiving, or output, transducer. Another approach is to use unidirectional transducers wherein the wave energy is launched or received in one direction for the input and output transducers. When an anisotropic material, such as ST quartz, serves as the piezoelectric substrate for the surface acoustic waves, the propogation characteristics are stable in one direction of transmission but not in the other generally orthogonal direction. In the latter situation, the ST quartz exhibits a temperature dependent, preferential direction of transmission which produces a temperature dependent signal attenuation in the operation of the SAW filter. This phenomemon is well known as discussed, for example, in a paper entitled "Phase Errors in Long Surface Wave Devices" by D. T. Bell, Jr. at page 420 of the I.E.E.E. Group on Sonics and Ultrasonics, Proceedings of Astrosonics Symposium, October 1972.
Unidirectional transducers are typically arranged so that the directional characteristics of the input and output transducers are aligned for being coupled together by launched surface waves travelling along a straight path. This straight path may then be advantageously oriented in the direction on the substrate surface exhibiting the most stable propagation characteristics during temperature changes. However, it is advantageous to change the direction of propagation of the surface waves on the piezoelectric substrate. For example, the change in direction increases the path length available from a given substrate size and is a requirement where density is a major consideration. One solution would be to house the SAW filter in a temperature controlled oven, but the additional control circuitry and the physical presence of the oven are costly and tend to defeat the high density objective. Conventional techniques accordingly do not provide a satisfactory solution to the introduction of the foregoing temperature dependent instability. It would be highly desirable to change the direction of propagation of surface waves in SAW filters in a manner wherein the effect of temperature dependent instability and temperature dependent signal attenuation are fully compensated.